1. Field of the Invention
This invention relates to rubber compositions and, more particularly, to a rubber composition that can be used to define a component of a power transmission belt.
2. Background Art
V-ribbed power transmission belts are used extensively in the automotive industry to drive various systems in engine compartments. As examples, these types of belts are commonly utilized to drive air compressors, alternators, etc.
It is common with this type of belt to embed, in ribbed portions of the belt, short fibers such as cotton, polyamide, vinylon, rayon, aramid, etc., with the lengths of the fibers aligned generally widthwise of the belt. The fibers provide lateral reinforcement to the belt body. It is also known to project the fibers from pulley-engaging side surfaces of the belt to controllably alter the frictional characteristics between the belt and cooperating pulley. At the same time, the projecting fibers suppress sound generation caused by adhesion between the belt and cooperating pulleys.
However, by increasing the lateral strength of the belt body through the incorporation of the reinforcing fibers, the ability of the rubber layer to elongate in the lengthwise direction of the belt may be decreased. The reinforcing fibers are commonly embedded in the compression rubber layer which experiences the decreased elongation capabilities. As a result, the bending properties of the reinforced rubber are generally adversely affected. By decreasing bending resistance, there is a tendency of the rubber to crack prematurely in normal operation. This cracking phenomenon is particularly prevalent in systems wherein the belt is caused to be severely bent in a serpentine manner and its outside surface is used to engage cooperating pulleys in use.
The recent trend in the automotive industry has been to design engines with greater displacement. Further, in the interest of reducing gas consumption and harmful emissions after combustion, these engines have been designed to operate in a lean combustion mode. Thus, there is a tendency of these engines to vibrate more significantly during operation than the conventional designs that preceded them. The power transmission belts operating on these engines are thus subjected to more severe vibratory conditions.
Further, these belts are typically operated in more compact engine compartments, thus requiring that they be trained around relatively small diameter pulleys at severe angles through a serpentine path. Thus the belts become heavily loaded and tend to generate a significant amount of sound during use. It is generally believed that the cause of the sound generation is a result of a repeated stick-slip phenomenon that occurs between the belt and cooperating pulleys.
To diminish the effects of this stick-slip phenomenon, it is known to apply powder, such as talc, to the pulley-engaging portions of the belt. It is also known to adhere silicone oil thereto. This surface treatment is intended to decrease the coefficient of friction between the engaging surfaces on the belt and pulleys. One method for treating belts in this manner is shown in JP-UM-B-7-31006.
In JP-A-5-132586, it is described that a silicone oil adsorbed in porous particles, such as activated carbon, on the exposed side surfaces of the belt, will keep the frictional coefficient between the belt and pulleys at a reduced, constant value over an extended operating period.
As another alternative, JP-A-2001-173728 discloses the blending of a solid, inorganic lubricant in a power transmission belt component to control frictional characteristics of the pulley-engaging surfaces thereon.
While applying talc powder or silicone oil to the pulley-engaging surfaces of the belt reduces stick-slip sound generation, generally this positive effect will not last for an adequate, extended period. During the initial stages of use, the coefficient of friction between the belt and pulleys may be adequately reduced. However, after extended running periods, the amount of lubricant/powder may diminish so that the effects thereof on the frictional characteristics of the belt likewise diminish unacceptably.
While the blending into the belt of porous particles with adsorbed silicone oil has extended the positive friction reducing effect over a more substantial period, there are inherent problems associated with making power transmission belts in this manner. It is difficult to uniformly blend and disperse the porous particles throughout the rubber in order to have the oil bleed uniformly and in an adequate amount at the exposed belt surfaces. Further, generally the porous particles only immediately at, or adjacent to, the exposed belt surfaces, bleed appreciably to significantly affect the frictional characteristics. Little, if any, bleeding of oil from any appreciable depth below the exposed surfaces may result.
It is also believed that the blending of a solid lubricant has limitations in terms of its long term effectiveness. Adequate reduction of the coefficient of friction between a belt and cooperating pulley may not be realized with belt constructions incorporating short reinforcing fibers.